1. Field of the Invention
The subject invention generally pertains to induction motors and generators, and more specifically to nondestructive testing of their rotors.
2. Description of Related Art
Induction motors typically include a rotor that rotates in response to a rotating magnetic flux generated by alternating current in a stator associated with the rotor. A rotational speed differential (known as xe2x80x9cslipxe2x80x9d) between the rotor and the rotating flux induces a current through a rotor cage. A rotor cage typically consists of a single aluminum casting having several conductive bars that run axially through the rotor and are joined at each end by two conductive end rings. Current induced in the bars creates a magnetic flux that opposes that of the stator, thus providing the rotor with rotational torque.
Sometimes it is desirable to inspect the integrity of a rotor before a new motor is assembled or before considering its use in a rebuilt motor. It is especially valuable to know the impedance of each rotor bar to identify rotor faults such as a cracked bar, separation between a bar and an end ring, or porosity of a bar and/or end ring. However, inspecting and identifying such faults is difficult to do, as cast aluminum cages are often cast directly into a laminated steel core of the rotor.
To provide a nondestructive test for rotors, an apparatus and method disclosed in U.S. Pat. No. 3,875,511 exposes a rotating rotor to what appears to be a constant magnetic field provided by an electromagnetic sending coil. The rotor bars crossing the magnetic lines of flux induce a current through the bars. A receiving coil detects the induced current to provide an analog signal that can be displayed on an oscilloscope.
An analog display, however, can be difficult to interpret and quantify. For example, in some cases the spacing between adjacent bars is so close that the spikes or peaks of an analog signal may tend to run together, thus making it difficult to distinguish one spike or bar from another. Similar negative results may occur when the bars are slightly recessed below the outer periphery of the laminated core. In such cases, portions of the core overlaying a bar may adversely shield the bar from a sending or receiving coil, and thus reduce the amplitude of the sensed signal. Also, when the bars are hidden below the outer surface of the core, a simple analog display may not provide a clear indication of how many bars are actually in the rotor. Manufacturers of new rotors will, of course, know how many bars are in their own rotors; however, for those that rebuild motors manufactured by others, the number of bars may be unknown.
With an analog display, electrical noise or a stray spike could be misinterpreted as another bar. Moreover, with an analog display, it can be difficult to establish the repeatability of the readings. Repeatability or comparison of one set of readings to a later one can be valuable not only to establish the credibility of a particular set of readings, but also to determine whether a rotor is deteriorating over an extended period of use.
To quantify the integrity of a rotor of an induction motor or generator, it is an object of the invention to nondestructively create a digital signature that indicates the impedance of each bar of the rotor.
Another object of the invention is to repeatedly check the impedance of each bar of a rotor to establish a credible record of the rotor""s integrity.
Another object is to create and store a digital record that indicates the integrity of a rotor, and later reference that record to determine the extent that the rotor may have deteriorated over an extended period of operation.
Another object is to determine the number of bars in a rotor by sequentially sensing the impedance of each bar for more than a full revolution of the rotor to create a repeating pattern that indicates that every bar has been checked at least once.
A further object of the invention is to create digital raw data that indicates the impedance of a rotor""s bars, and to manipulate the data by way of a microprocessor to create an enhanced visual indication of the impedance of each rotor bar.
A still further object is to induce an electrical current in a rotor by varying the current in an electromagnetic sending coil.
Another object is to sense the current through an electromagnetic sending coil to acquire an indication of a rotor bar""s impedance.
Yet another object is to sense the current or voltage of electromagnetic receiving coil to acquire an indication of a rotor bar""s impedance.
Another object is to distinguish one fault from another, wherein one fault is one or more bars having impedance that exceeds a predetermined limit, and another fault is a pattern of gradually varying impedance or a number of bars of especially high impedance being unequally distributed around the rotor.
These and other objects of the invention are provided by rotor analyzer that exposes the bars of a rotor to a varying magnetic field to induce a current through the bars. A digital signal is created that varies as a function of the induced current. A microprocessor manipulates the digital signal to provide an enhanced visual indication of the impedance of each rotor bar.